The team operating the reactor monitored the condition and confirmed the reactor's safe shutdown at about 4 p.m., the operator said in a statement.

The power plant had been operating continuously for 383 days.

"Our plant produces more than a million dollars worth of power every day," Plant Manager Tom Lynch said. "Obviously we are anxious to get the system repaired and bring the plant back on line as soon as possible."

He added it wouldn't restart until crews "have fully identified and corrected the reasons" for the shutdown.